Google's previous update for the Chromecast with Google TV fixed the annoying recovery screen error which, in certain cases, would require a reset to get the device working again. With that bug firmly in the past, a new update mainly focuses on improving the device and delivering some new functionality.
The update, version QTS1.210311.005, is still based on Android 10 (via 9to5Google) but it includes the April security patch. It also introduces new Advanced Video Controls settings, which will allow users to control HDR formats, resolution/refresh rate, and color formats. Another notable change is that HDMI-CEC can now finally be configured to turn on/off only the TV.
Google is also introducing some connectivity enhancements with the new update. The device should work better with 5GHz and mesh networks and audio stutters over Bluetooth should also be minimal. Lastly, there are HDMI hotplug improvements that help the Chromecast with Google TV detect the best display settings available.
The new update is already being rolled out and you should receive it soon. You can check for it by tapping on your profile picture and heading to Settings > System > About > System update.